1. Field of the Invention
The present invention relates to a receiver timing recovery circuit in a communication system employing the scheme of modulating the amplitude of a sine wave or a pulse train as a carrier wave.
2. Description of the Related Art
In a communication system in which a transmit signal created by modulating the amplitude of a sine wave or a pulse train as a carrier wave is transmitted from the transmitting end to the receiving end, the job that the receiving end is expected to do is to recover the original signal information. Conventionally, when sampling a high-frequency received signal in an analog/digital converter (hereinafter, sometimes referred to as an ADC), the signal is sampled at a frequency several times higher than the Nyquist frequency. This is known as oversampling. The higher the sampling frequency, the higher signal-to-noise ratio (S/N ratio) that can be achieved, but the amount of computation increases.
Undesired noise components occurring in the frequency range higher than the signal bandwidth as a result of the oversampling are removed by passing the output signal of the ADC through a low-pass filter (LPF). Then, the oversampled data is converted back to the original sampling rate; this operation is called “decimation”. The decimation is accomplished using a decimator whose configuration is shown in FIG. 1. As shown, the decimator comprises a low-pass filter (called the decimation filter) and a downsampler. In the figure, Fs indicates the sampling frequency, and D designates a positive integer.
Here, the case where a timing recovery process is provided in addition to a data reconstruction process in the above communication system is considered. As the timing information is contained in the received signal, conventionally, the timing information is recovered by using a portion of the received signal converted back to the original sampling rate after oversampling. More specifically, the amplitude values of the samples adjacent on both sides of the sample used for signal decision are found, and the timing information is obtained from their difference. However, as noise is almost always superimposed on the received signal, it is difficult to extract accurate timing information from one single signal.
In Japanese Unexamined Patent Publication No. H03-098309, herein cited as a prior art document that has relevance to the present invention, there is disclosed a signal distortion correcting method for use in a digital subscriber transmission interface, wherein a received signal from a transmission line is converted by an oversampling A/D converter into high-speed digital data, and the resulting digital data sequence is passed through a plurality of low-pass comb filter stages in a decimation filter to remove frequency components higher than the basic sampling frequency, after which the data is converted, by decimation, into a data sequence operating at the basic sampling rate and, after performing echo cancellation, the transmission loss occurring due to the low-pass comb filters and the loss distortion due to the transmission line are corrected simultaneously by a digital equalizer operating at the basic sampling rate.